Embodiments of the invention relate generally to magnetic resonance (MR) imaging and, more particularly, to reducing interference between MR coil elements of a phased array.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Often, a phased array is used during MR imaging. A phased array includes a plurality of radio-frequency (RF) coils or coil elements, and, conventionally, each RF coil element of the phased array is tuned to the same frequency. Generally, the resonance frequency of the system is chosen as the operating frequency of each coil element of the phased array. That is, the coil elements of the phased array are generally tuned to a frequency at which a whole-body transmit coil, transmit head, or the like operates to maximize reception. As such, the coil elements are at a resonant frequency with the whole-body RF coil or other transmit coil. Typically, coil elements tuned in this manner are “turned off” during operation of the MR system's transmit coil or other transmit coil to avoid resonance that can cause interference and degrade SNR and/or image quality.
RF coil elements of the array are generally configured or arranged to minimize interference, which can be caused by cross-talk between RF coil elements. For example, interference may be caused by inductive coupling, where one coil element inductively induces a current into an adjacent coil element. Inductive coupling becomes more predominant as coil density or coil channel count increases. Generally, inductive coupling tends to increase correlated noise between coil elements of an array. As such, the signal-to-noise ratio (SNR) of each coil generally degrades, thus degrading the performance of the RF coil array. Cross-talk interference may also be caused by inductive coupling between transmit and receive coils.
There are a variety of known techniques implemented to isolate cross-talking among coil elements. For example, to minimize cross-talking between adjacent coil elements, techniques that utilize critical or geometric coupling (e.g., minor overlap), inductive coupling, and capacitive coupling have been employed. However, geometric decoupling such as overlap, inductive coupling, and capacitive coupling can limit some effective geometric designs due to spatial constraints imposed by such techniques.
Another technique used to minimize cross-talk between non-adjacent coil elements employs low input impedance Pre-amplifiers (Pre-amps). The low input impedance Pre-amps are generally used to de-resonate each RF coil to reduce RF current of each coil and to reduce induced RF current between RF coils, thus improving isolation between coil elements of the array. With such techniques, RF coils are generally tuned to a resonance frequency (thus the need to de-resonate) and matched via a matching network to a 50-ohm output. The low input-impedance Pre-amp transforms the 50-ohm output to a high impedance around 1 k ohms to meet an optimum source impedance of the Pre-amps in order to yield a low noise figure for optimal SNR. However, low-input Pre-amps can generate limited blocking impedance due to stability concerns associated with the Pre-amps and coil size/loading dependency.
It would therefore be desirable to have a system and method capable of minimizing interference associated with RF coil elements of a phased array while overcoming the aforementioned drawbacks.